Crystalline ZSM-5, and its conventional preparation using tetrapropylammonium cations as a structure directing agent, are taught by U.S. Pat. No. 3,702,886 and U.S. Pat. No. Re. 29,948, the entire disclosures of which are incorporated herein by reference. Conventional ZSM-5 has a distinctive X-ray diffraction pattern which distinguishes it from other known crystalline materials and is a highly versatile catalyst useful in a variety of organic conversion reactions.
For some acid-catalyzed reactions over zeolites, it is beneficial to reduce diffusion lengths of the reagent and/or product molecules by employing a zeolite with a reduced crystal size and hence an increased external surface area. This may have the effect of reducing the shape selective effects of the zeolite, but for reactions that require only strong activity this may not be important.
An example of small crystal ZSM-5 is disclosed in U.S. Pat. No. 5,240,892, in which the ZSM-5 is in the form of platelets having first and second major dimensions of at least about 0.05 micron, preferably at least about 0.1 micron, and a minor third dimension of less than about 0.02 micron, preferably less than about 0.01 micron. The ZSM-5 has a mesitylene sorption capacity of at least 3.0 weight %, and is produced using precipitated silica as the silica source either in the absence of an organic directing agent or using n-propylamine as the directing agent.
In addition, in Chem. Mater. 21 (2009) 641-654, D. Serrano et al. claim synthesizing ZSM-5 crystals as small as 5 to 10 nm using a dual template of tetrapropylammonium (TPA) ions and phenylaminopropyltrimethoxysilane. In this method, the silanizing agent is introduced after the synthesis gel is pre-heated for short periods of time before the onset of zeolite crystallization. FIG. 1 of Serrano et al. shows a schematic representation of the crystallized products, whereas FIG. 2 shows TEM images of the product. Although these TEM images show very small particles, the peaks in the powder XRD of the product from this work are not as broad as would be anticipated for 5 to 10 nm crystals.
Ryoo and coworkers have reported in “Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived, catalysts”, Nature 461, 246-249 (10 Sep. 2009), the synthesis of a single unit cell-thick version of ZSM-5 by using a single templating agent composed of a 22-carbon atom alkyl chain and two quaternary ammonium groups separated by a methylene chain of 6 carbon atoms. Here the quaternary ammonium groups are located within the single-unit cell nanosheets, which are separated from one another by the long alkyl chains. FIG. 3 of Ryoo et al. shows a schematic of the unilamellar and multilamellar version of the ZSM-5 crystals that are a single unit cell in thickness.
A common problem in the isolation of many nanocrystalline products is that their aggregates are so small that the products must be collected by centrifugation rather than by simple filtration techniques. Large scale-ups are therefore difficult to process.
According to the present invention, it has now been found that new ultra-small crystal forms of ZSM-5 can be produced. These ultra-small crystal forms of ZSM-5 have unique powder XRD patterns and are also referred to herein as EMM-20. EMM-20 can be made across a wide range of Si/Al ratios (infinity to 10) and is generally produced in highly aggregated form allowing its collection by standard filtration techniques. EMM-20 may be useful as a catalyst in reactions where high acid activity and large surface area are important.